“There’s a very clear lesson here. What it shows is that if you make a cleaner energy source cheaper, you will displace dirtier sources,” Roger Pielke Jr., a climate expert at the University of Colorado, told the Associated Press.

The study also pointed to slower economic growth as being responsible for the reduced CO2 emissions. In 2011, the U.S. GDP increased by 1.8 percent, but emissions decreased by 2.4 percent, which suggests that the carbon intensity of the economy declined by about 4.2 percent.

The U.S. has reduced its carbon dioxide emissions more than any other country during the last six years, the International Energy Agency has said.

In 2007, America's CO2 emissions were at an all-time high of about 6 billion metric tons. Projections for 2012 are around 5.2 billion, almost meeting the 1990 levels of 5 billion, according to the AP.

However, the United States' carbon emissions are still some of the highest in the world, according to data from the World Bank: In 2008, the U.S. was emitting 17.8 metric tons of CO2 per capita, compared to the world average of 4.76 metric tons.